This invention relates to recovery of polyester from waste materials, to the preparation of adhesive materials from the recovered polyester and, in particular, to hot-melt adhesives of exceptionally high tack characteristics. Commercially available hot-melt adhesives can be used in packaging, bookbinding, and disposable soft goods markets such as backing, labels and decalcomania. Additional application areas include laminates, seaming tapes and related applications.
Polyesters, typically polyethylene terephthalate, are enjoying an increasing demand in the manufacture of not only fibers and films but also of containers for food and drink and of food packaging films owing to their favorable mechanical properties, transparency, heat resistance, electrical characteristics, and the like. Since such polyester containers and films, to a certain extent, function as gas barriers, their use is favorable for the preservation of food and drink, as well as other materials requiring such barriers. However, the properties of polyesters, which cause their widespread use, also result in the creation of vast quantities of polyester waste. The waste is generated as a by-product of the manufacturing process, i.e., as by-products from extrusion and forming of the polyester resin into finished articles and from the disposal of the used articles. Polyethylene terephthalate polymers are accordingly widely available as scrap photographic film and synthetic fibers, as sludges obtained as clean-up by-products from manufacturing plants, and as contaminated scrap from polyethylene terephthalate waste recovery sources such as bottle reclaiming plants.
For economically using the polyester wastes, there have been known methods, such as (1) utilizing the polyester wastes as they are for some purposes, (2) depolymerizing the polyester wastes to give starting materials useful for production of new polyesters, and (3) reacting the polyester wastes with polyols or polycarboxylic acids to give new polyesters.
Depolymerizing waste polyethylene terephthalate polyester by glycolysis of the polyester with excess ethylene glycol in the presence of water is taught in U.S. Pat. No. 4,078,143. Recovery of polyester waste with excess ethylene glycol at elevated temperatures by glycolysis of the polyester forms bis(2-hydroxyethyl) terephthalate and low molecular weight polymers thereof. Formation of glycol ethers, principally diethylene glycol, which copolymerize with the bis(2-hydroxyethyl) terephthalate, is a disadvantage of this method. Formation of the diethylene glycol can be inhibited by use of lithium acetate dihydrate in combination with zinc acetate dihydrate and/or antimony trioxide, as taught in U.S. Pat. No. 3,830,739. However, the results obtained in both U.S. Pat. No. 4,078,143 and 3,830,739 indicate the presence of diethylene glycol in the recovered polymers.
Other known digestion processes include treatment with low molecular weight polyols selected from the class consisting of diols and triols (U.S. Pat. No. 4,048,104); treatment with caustic monohydric alcohol (U.S. Pat. No. 3,728,287); treatment with monohydric alcohol and ester interchange catalyst (U.S. Pat. No. 3,488,298); heating with glycols and vinyl acetate (U.S. Pat. No. 3,857,799); heating with bis ester solvents (U.S. Pat. No. 3,884,850); and dissolving in ethylene glycol and/or terephthalic acid and/or dimethyl terephthalate (U.S. Pat. No. 3,907,868; 3,701,741; 3,703,488; 3,776,945; 3,257,335). The polyesters are taught as digested for reuse in the process of making additional polymers of molecular weights in excess of 15,000, that is, the digestion products are reintroduced into the process from which they originated or into other processes as a polyol ingredient to prepare other polymers.
Reacting the polyester wastes with polyols and/or polycarboxylic acids in the presence of a titanium catalyst to produce an oligomer and then polycondensing the resulting depolycondensation product with at least one polycarboxylic acid or an anhydride thereof and at least one polyol is taught in U.S. Pat. No. 3,951,886.
As noted hereinabove, the several methods for recovering scrap polyethylene terephthalate polyester to obtain new polyesters can result in the production of undesirable by-products, such as diethylene glycol, which contaminate the final product or require further purification before preparation of the new polyesters. Such processes can be costly and represent a significant increase in the cost of recovering scrap polyethylene terephthalate, even to the extent of causing the recovery process to be uneconomical in view of the comparable cost of producing virgin polyethylene terephthalic resin.
Accordingly, it is an object of this invention to provide a low-cost process for the recovery of polyethylene terephthalate waste wherein the product resulting therefrom is directly useful without further purification to remove undesirable by-products before preparation of new polyesters.
It is further an object of this invention to provide a process for the preparation of hot-melt and pressure-sensitive adhesives from polyethylene terephthalate waste which adhesives have superior properties as compared with currently available hot-melt adhesives and pressure-sensitive adhesives.
A further object of this invention is to provide a unique method for digesting polyester waste of polyethylene terephthalate which minimizes the production of by-products such as ethylene glycol and diethylene glycol.